Add cursorSupportedByOplogTailing.
In "meteor run", run mongo in repl-set mode, so that oplog is available.

Note: this is pretty slow. Before merging this branch, should cache the fact
that replset has been initiated on port N so that on the next run, if the port
hasn't changed, we don't need to re-initiate.
Factor tailing code out of observe-changes code.
Don't reset the replset unless the port changes.

(Resetting the replset is slow!)